Siloxane polymers have unique properties derived mainly from the physical and chemical characteristics of the siloxane bond. These properties include low glass transition temperature, thermal and oxidative stability, resistance to ultraviolet radiation, low surface energy and hydrophobicity, high permeability to many gases, and biocompatibility. The siloxane polymers, however, often lack tensile strength.
The low tensile strength of the siloxane polymers can be improved by forming block copolymers. Some block copolymers contain a “soft” siloxane polymeric block or segment and any of a variety of “hard” blocks or segments. Polydiorganosiloxane polyamides, polydiorganosiloxane polyureas, and polydiorganosiloxane polyoxamides are exemplary block copolymers.
Polydiorganosiloxane polyamides have been prepared by condensation reactions of amino terminated polydiorganosiloxanes with short-chained dicarboxylic acids. Alternatively, these copolymers have been prepared by condensation reactions of carboxy terminated polydiorganosiloxanes with short-chained diamines. Because polydiorganosiloxanes (e.g., polydimethylsiloxanes) and polyamides often have significantly different solubility parameters, it can be difficult to find reaction conditions for production of siloxane-based polyamides that result in high degrees of polymerization, particularly with larger homologs of the polyorganosiloxane segments. Many of the known siloxane-based polyamide copolymers contain relatively short segments of the polydiorganosiloxane (e.g., polydimethylsiloxane) such as segments having no greater than about 30 diorganosiloxy (e.g., dimethylsiloxy) units or the amount of the polydiorganosiloxane segment in the copolymer is relatively low. That is, the fraction (i.e., amount based on weight) of polydiorganosiloxane (e.g., polydimethylsiloxane) soft segments in the resulting copolymers tends to be low.
Polydiorganosiloxane polyureas are another type of block copolymer. Although these block copolymers have many desirable characteristics, and involve a one pot preparation, some of the polydiorganosiloxane polyureas tend to degrade when subjected to elevated temperatures such as 250° C. or higher.
Polydiorganosiloxane polyoxamides are another type of block copolymer, wherein the relatively unstable urea link of the polydiorganosiloxane polyurea is replaced with a UV and thermally stable amide link. However, the process of making these polydiorganosiloxane polyoxamides comprises an intermediate isolation and/or purification step, where a polydiorganosiloxane diamine is first treated with a molar excess of diethyl oxalate to cap-off the amine ends and then the unreacted excess is removed. Then, the isolated prepolymer is reacted with a stoichiometric amount of a short chain diamine to get the targeted polydiorganosiloxane polyoxamides.